Spectral and Spatial Heterogeneity in Molecular Assemblies of Amino Styryl Pyridinium Dyes
Lusk, Amy Louise
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https://hdl.handle.net/2142/84026
Description
Title
Spectral and Spatial Heterogeneity in Molecular Assemblies of Amino Styryl Pyridinium Dyes
Author(s)
Lusk, Amy Louise
Issue Date
2001
Doctoral Committee Chair(s)
Bohn, Paul W.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Language
eng
Abstract
The aggregation properties of amino styryl pyridinium dyes (ASPs) greatly reduce their ability to be incorporated into molecular scale devices that would utilize their large molecular hyperpolarizabilities, in such areas as molecular optics, opto-electronics, or telecommunications. This work correlates aggregation in molecular assemblies of ASPs to the resultant linear optical properties and two dimensional phase behavior. Aggregation of the ASP, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, I, can be detected spectroscopically by the appearance of blue shifted absorption bands and a red shift of the emission band. The far field fluorescence of Langmuir-Blodgett (LB) films of I is spectrally heterogeneous displaying bands assigned to monomer and various aggregated (n-mer) species. Band assignment is based upon the interaction integral, which predicts the perturbation to the monomer excited state energy by interaction with neighboring molecules. This work is the first to identify a distribution of aggregate sizes in thin films of I. Spatial heterogeneity visualized by epi-fluorescence microscopy reveals a morphology of coexisting liquid expanded (LE), liquid condensed (LC), and locally collapsed phases. The large spectral shift associated with aggregation of I allows its electronic spectra to provide the contrast in fluorescence microscopy images of LB films. Epi-fluorescence emission images thus reveal the aggregation-induced electronic structure of the molecules composing the phase domains. Correlation of film and solution fluorescence spectra with these emission images suggest that the LC domains are composed of dimers, trimers and larger n mers up to the fully extended aggregate, while the LE phase is predominantly populated by monomers with some fully extended aggregates. Furthermore, domain size and shape were found to be sensitive to fabrication conditions such as; spreading solution concentration, compression speed, aging, and subphase composition, but insensitive to deposition pressure. Far-field fluorescence from films of I is nearly constant as a function of pressure, indicating aggregation occurs long before the compression process, perhaps even persisting from the spreading solution. It was shown that the number of intermediate aggregate species observed in fluorescence spectra and the number of phases observed in epi-fluorescence images are intimately related to molecular structure through the strength of the excited state intermolecular coupling, which is responsible for aggregation.
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